Adipic acid is commercially produced by the oxidation of cyclohexanol and cyclohexanone by concentrated nitric acid. Initial product recovery and purification are accomplished through crystallization of the reaction mixture followed by solid-liquid separation. Additional purification of adipic acid is accomplished through one or more steps of aqueous recrystallization followed by solid-liquid separation. The solid cake from the solid-liquid separation unit after the final recrystallization step typically contains about 3 to 12 wt % water. This water is removed in a subsequent drying step in which heat is supplied to convert the water into vapor that is separated from solid adipic acid. A flow of gas is often used as the carrier of heat to be supplied to the moisture, as well as the carrier of the vapor to be removed. A typical desired moisture content of the dry adipic acid product is less than 0.2 wt %.
In the solid cake to be dried (as described above) the moisture is present in two forms: surface moisture (free moisture) and intrinsic (bound) moisture (including moisture present in liquid inclusions). Although the removal of free moisture is easy and fast, removal of bound moisture is difficult and slow. Therefore, practical drying processes usually use long residence time and high temperature in order to remove enough moisture so that the final product meets specification. However, at high temperature drying results in the formation of large amount of fine particles, presumably due to the dissolution of significant solid from particle surfaces by water prior to its vaporization. The dissolution should be more prominent if the moisture content of the moist adipic acid is high (e.g., 10 to 12%). Presence of so-called fines (very small particles) negatively impacts the loading-unloading characteristics of the product. It would therefore be desirable to have a drying process that would produce a product that meets moisture specification without forming excessive fine particles.
The present invention provides such a method of drying. In the present invention at least a portion of the water or more specifically at least a potion of the free moisture contained in moist adipic acid is removed by evaporation at low temperature prior to evaporating at high temperature the remaining water to be removed. Initial removal of water at low temperature should reduce dissolution and fines formation and drying at high temperature should remove the difficult to remove water contained in the solid.